CHAPTER 9 - FIELD EFFECT TRANSISTORS

Abstract

This chapter discusses the field effect transistors (FETs). The chapter describes that it is possible to make semiconductor devices in which the output current is controlled directly by an applied voltage; these are known as field effect transistors. There are two basic types of FET available—the junction FET (JFET) and the insulated-gate FET (also known as a metal-oxide semiconductor FET, or MOSFET). The junction FET consists of a single bar of either N or P-type semiconductor material, through which the output current to be controlled flows. The MOSFET, like the JFET, consists of an N or P-channel conductor from drain to source. Four types of MOSFET can be made: N and P-channel depletion types, which are similar to the corresponding JFETs in operation, and N and P-channel enhancement types, whose channels are non-conducting unless enhanced by forward bias on their gates. The chapter describes that in their present state of development, FETs are less satisfactory as amplifiers than are bipolar transistors. A FET (often a MOSFET) may be used to gate a signal voltage.